TCl and ICl each have been synthesized in continuous or batch processes which involve the reaction of phosgene with terephthalic acid (TPA) or isophthalic acid (IPA) in the presense of an equimolar complex of phosgene and dimethylformamide (DMF). Parker et al. in U.S. Pat. No. 3,184,506 disclosed that it is necessary in a continuous process to maintain some unreacted TPA or IPA in the reaction zone and the product stream; otherwise, the DMF/phosgene complex that catalyzes the phosgenation reaction will degrade. TPA and IPA can be manufactured by the oxidation of p-xylene and m-xylene respectively. Those reactions results in the production respectively of 4-carboxybenzaldehyde and 3-carboxybenzaldehyde (collectively CBA) as intermediates. Because of incomplete oxidation of those intermediates, 4-carboxybenzaldehyde and 3-carboxybenzaldehyde, respectively, are found in the TPA and IPA so-produced in amounts generally in the range between about 200 and 2000 ppm. When CBA-containing TPA or CBA-containing IPA is reacted with the DMF/phosgene complex, CBA is converted, respectively, to 4-formylbenzoyl chloride or 3-formylbenzoyl chloride (collectively FBC). Reaction of FBC with the DMF/phosgene complex gives alpha,alpha-dichloro-4-toluoyl chloride or alpha,alpha-dichloro-3-toluoyl chloride (collectively DCTC).
In accordance with the present invention, it has been found that even when present in small quantities, both FBC and DCTC cause unwanted color formation when either TCl or ICl containing the same is used to form a polymer. Such coloration is frequently unacceptable, particularly in respect of high performance engineering plastics, e.g. polyphenolate/carbonate. So as to avoid color formation in such polymers, the polymer manufacturer may require TCl or ICl having less than 100 ppm, preferably less than 50 ppm of FBC and/or DCTC.
Neither the process for oxidizing p-xylene nor that for oxidizing m-xylene can be operated economically to produce TPA or IPA having a CBA content of 100 ppm or less. One or more steps must be added to the process for manufacturing TPA or IPA so as to purify the acid. For example, IPA has been crystallized from acetic acid, a costly corrosive process. High purity TPA has been produced by catalytically reducing CBA to p-toluic acid and removing the latter by crystallization, also at added cost. Removal of FBC and DCTC from TCl or ICl by crystallization and/or distillation are costly undertakings requiring substantial investment, particularly for solvent recovery facilities. Furthermore, the prior art has failed to provide a procedure whereby TPA or IPA can be reacted with the DMF/phosgene complex so as to produce TCl or ICl having less than 100 ppm of FBC and/or DCTC. Conversion of CBA in the reaction with the phosgene/DMF complex is essentially quantitative. Consequently, when operating in accordance with the prior art, the aggregate amounts of FBC and DCTC found in the TCl or ICl will be directly proportional to the amount of CBA found in the TPA or IPA starting material.